1. Field of Invention
The present invention relates a method of forming a semiconductor device. More particularly, the present invention relates to a method of forming an interconnect having a stacked alignment mark.
2. Description of Related Art
Photolithography is a crucial process for the process for manufacturing the semiconductor device. In the conventional process for manufacturing a device, depending on the manufacturing complexity of a device, it is necessary to perform the photolithography for about 10 to 18 times. In order to correctly transfer the patterns on the photo mask onto the wafer, before the exposure process of each photolithography process is performed, it is necessary to perform an alignment process for aligning the film layer to each other so that the improper pattern transfer will not happen.
Typically, the alignment mark is formed on the wafer for forming scattering site or diffraction edge during the alignment process. Hence, while a light source is provided to illuminate the wafer, the diffraction patterns caused by the light beam passing by the alignment mark are reflectively projected onto the alignment sensor or onto the first-order diffraction interferometer alignment system.
However, in the semiconductor process, there exist some problems in aligning the film layers to each other. For example, while aligning an alignment mark of a dielectric film layer over a substrate, since there is another dielectric film layer located under the alignment mark, a portion of the light passing through the alignment mark also pass through the lower dielectric film layer. Therefore, the reflect beam does not reflect to the alignment sensor. Hence, the alignment result is poor. That is, the misalignment happens so that the alignment accuracy between the film layers is affected.